Conductive-ring ferromagnetic marker and method and system for using same

ABSTRACT

A ferromagnetic marker is used in tagging objects to allow selective detection of the tagged objects within an interrogation zone having a magnetic field varying at a fundamental frequency. The marker comprises a ring-shaped electrical conductor for carrying a current induced by the varying magnetic field. An element which is non-linearly polarized in response to an electromagnetic field is disposed proximate the conductor to modulate the current flowing therethrough. A modulated electromagnetic signal is then generated and sensed to allow detection of a tagged object.

[ Aug. 21, 1973 United States Patent [191 Fearon 325/8 X 343/65 SS343/65 SS [54] CONDUCTlVE-RING FERROMAGNETIC 3,098,971 7/1963Richardson.........................

MARKER AND METHOD AND SYSTEM FOR 3,218,638 11/1965 Homg USING SAME3,384,892 5/1968 [75] Inventor: Edward R. Fearon, Tulsa, Okla.

Primary Examiner-David L. Trafton 73 A t 1 It 1 .D u 1 sslgnee gf met nematlona Inc a as Attorney-D. Carl RlchardsJerry W. Mills et al.

[22] Filed: Nov. 24, 1971 ABSTRACT [21] Appl. No.: 201,858

Related U.S. Application Data [60] Division of Ser. No. 747,050, March22, 1968, Pat.

A ferromagnetic marker is used in tagging objects to No. 3,631,442,which is a continuation-in-part of Ser. allow selective detection of thetagged objects within No. 680,666, Nov. 6, 1967, abandoned.

an interrogation zone having a magnetic field varying at a fundamentalfrequency. The marker comprises a [52] U.S. 340/280, 325/8, 340/258 R,

ring-shaped electrical conductor for carrying a current induced by thevarying magnetic field. An element which is non-linearly polarized inresponse to an electromagnetic field is disposed proximate the conductorto modulate the current flowing therethrough. A modulatedelectromagnetic signal is then generated and sensed to allow detectionof a tagged object.

[56] References Cited UNITED STATES PATENTS 13 Claims, 9 Drawing Figures2,774,060 12/1956 Thompson 340/258 R PATENTED M1821 I973 SHEET 1 BF 5FIG. I

PATENTEDMISZI ms 3; 754' 226 SHEET 2 0F 5 FIG. 3

PATENTEBMJBZI I975 3754.226 sum 5 OF 5 FIG. 7

'; MECHANICAL l COIL SUPPORT 43 47 DOORWAY F CONDUCTIVE-RINGFERROMAGNETIC MARKER AND METHOD AND SYSTEM FOR USWG SAW CROSS REFERENCETO RELATED APPLICATIONS This application is a divisional application ofUS. Pat. application Ser. No. 747,050, filed Mar. 22, I968, now US. Pat.No. 3,63 l ,442, which was filed as a continuation-in-part of nowabandoned patent application Ser. No. 680,666 filed Nov. 6, 1967.

FIELD OF THE INVENTION This invention relates to a marker and a methodand system for detecting the marker for preventing unauthorized removalof objects having the marker attached thereto.

DESCRIPTION OF THE PRIOR ART There are in existence several systems fordetecting or preventing the theft of articles of value. One of thesecorresponding with US. Pat. No. 3,292,080, granted to E. M. Trikilis,Dec. 13, I966, makes use of a magnetometer and utilizes a magnetizedobject which identifies the article unless checkout procedure hasremoved the magnetism from the object. The magnetized object is attachedto or becomes a part of the merchandise or article of value, and byenergizing the magnetometer system as it passes through the doorway, isdetected. If the magnetized object has been demagnetized it causes nomagnetic signal as it passes through the doorway and is not detected.Demagnetizing is done in the process of checking out the merchandise.Thus by the checkout procedure an individual has free passage with themerchandise that has been paid for or recorded by the clerk. Anyadditional merchandise not paid for and however concealed radiates amagnetic influence, and energizes the magnetometer at the doorway,creating an awareness of security department personnel that something isbeing stolen.

Another system involves radioactive material which emits nuclearradiation. When the label containing the magnetic material is removedfrom the merchandise, the radiation is no longer emitted, and thereforeradiation detectors situated in the doorway are not energized. On theother hand, if the radiation emitters remain on the merchandise, doorwaysensors of nuclear radiation react, and security personnel are in aposition to prevent the theft.

In another system currently being employed in a mens wear department inMacy's in New York City, the operator uses a radio frequency generatingdevice embedded in a rubber pad. The radio frequency emitting device isfastened to the men's clothing, and if not removed, will energize radiofrequency detecting antennas at the doorway. In the nonnal course ofevents, when the merchandise is sold, a special fastener is unlocked andthe radio frequency emitter is removed from the clothing at the time itis sold, permitting the buyer to pass through the doorway withoutattracting the attention of the store detective.

All of the foregoing systems have severe difficulties of one kind oranother. The Trikilis system unfortunately requires a rather large pieceof ferromagnetic material for the marking of the merchandise. If toosmall a piece of ferromagnetic material is used, ambient variations inthe magnetic field are greater than the changes caused by the Trikilismerchandise marker. In

the case of the radioactive dot, there is a severe health probleminvolving danger to people from the nuclear radiation, and involvingdanger to those who remove the markers and store them. The system in usein Macy's Store unfortunately is limited by the extreme costliness ofthe radio frequency transmitter, and the limited period of time duringwhich its emission can be maintained by the little batteries with whichit is provided. True, larger radio frequency emitting pads could bemade, but these tear or injure the clothing, and are impracticallybulky.

SUMMARY OF THE INVENTION I have discovered a practical solution to theproblems presented but not solved by the workers in the prior art asdescribed above. As a matter of convenience, I choose to employelectromagnetic radiation. However, because of the inconvenience ofsupplying energy in a contraband marking, the energy to be radiated fromthe contraband marked device is delivered, instead, from structuralmembers of my sensing doorway.

I have found it extremely difficult to re-radiate or reflect energy in adistinctive manner from any merchandise marker for the reason that allsolid bodies and all electrically conductive masses (including the humanbody which is largely composed of salt water) also reflect or disperseelectromagnetic radiation and therefore must be considered in therecognition of any merchandise marking. A human being reflects moreelectromagnetic energy than any practical size of merchandise marker.

I have solved the problems just described by my discovery of anextremely simple device which can receive energy and re-emit it,receiving the energy in a frequency spectrum entirely distinct from thefrequency spectrum which is re-emitted. I do this by making use of theproperties of electrically and electromagnetically nonlinear systems. Ingeneral, it is the property of a nonlinear system that if a frequency Fis imposed at an energy level at which the nonlinearity of the systembecomes important, the system will generate frequencies 2F, 3F, 4F, etc.Similarly, if I impose on a nonlinear system signal sources whichdeliver approximately equal energy in each of two frequencies, thenonlinear system will generate other frequencies, not originallypresent. If the frequencies which I impose are F and F the nonlinearsystem will generate signals having frequencies'F, F F F F, 2P 2F, 2F,,and various other combinations of sums and differences of multiples ofthe frequencies which I impose.

In further accordance with the invention, a marker is disclosed whichwhen secured to an object enables detection of the object when theobject is in an oscillating electromagnetic field of an interrogationzone, such as a doorway, by radiating detectable electromagneticradiation in response to energy received from the oscillatingelectromagnetic field. The marker includes a ringshaped electricalconductor for carrying a current induced in the conductor by anelectromagnetic field the lines of flux of which link the conductor. Themarker also includes a substance which is nonlinearly polarized inresponse to an electromagnetic field and which is proximate to theconductor to modulate the current flowing in the conductor according tothe polarization of the substance when an electromagnetic field isinducing a current in the conductor and polarizing the substance. Themarker then radiates detectable electromagnetic radiation at apredetermined modulation of the oscillating electromagnetic field.

In accordance with another aspect of the invention, a system is providedfor detecting an object in an interrogation zone. The system includesmeans proximate the area for concurrently producing at least oneoscillating electromagnetic field in the zone. A marker is as sociatedwith each object to be detected for radiating detectable electromagneticradiation in response to energy received from the oscillatingelectromagnetic field, the marker comprising a ring-shaped electricalconductor for carrying a current induced in the conductor by anelectromagnetic field, the lines of flux of which link the conductor.The marker further includes a substance which is non-linearly polarizedin response to an electromagnetic field and which is proximate to theconductor to modulate the current flowing in the conductor according tothe polarization of the substance when an electromagnetic field isinducing a current in the conductor and polarizing the substance. Themarker thus radiates detectable electromagnetic radiation at apredetermined modulation of the oscillating electromagnetic field. Meansis also provided for sensing in the interrogation zone electromagneticradiation having the predetermined modulation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a somewhat diagrammaticview of a typical installation of the present system;

FIG. 2 is a view of a preferred embodiment of the conductive-ring markerof the invention;

FIG. 3 is a view of a variation of the marker shown in FIG. 2;

FIG. 41 is a view of another variation of the conductive-ring marker ofthe invention;

FIGS. A-B illustrate additional variations of the conductive-ringmarkers of the invention;

FIG. 6 is a block diagram of an embodiment of an energizing anddetecting system for use with the invention;

FIG. 7 is a diagram to assist in the explanation of the operation of theenergizing and detecting system; and

FIG. 8 is a diagram of a filter and coil system for use with the markerof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I now turn to FIG. I which is ageneral view of the manner in which my system operates in a store toprevent theft of merchandise. Merchandise l is provided with contrabandmarker elements 2. The checkout stand area 3 contains a deactivatingdevice 4 which is capable of changing the electromagnetic properties ofthe contraband marker elements 2. An energizing and detecting system Asituated in the 5B and 5C vicinity of the outgoing doorway 6 detects thecontraband marker elements 2, and identfies those which have not beensubjected to change at the checkout stand area 3 by the deactivatingdevice 4. In the use of my system, one way traffic, enforced by perphapsa turnstile 7, takes care of persons entering the store, prohibiting thecarrying of merchandise from the store to areas outside the store exceptthrough my outgoing doorway 6. The turnstile '7 is provided at the entryportal E.

I turn now to FIG. 2 in which there is illustrated anelectromagnetically functionable nonlinear marker device for use in thesystem shown in FIG. I. The ringshaped conductor 15 may be made, ifdesired, of a flat piece of metal, or if desired, may be composed of awire. In any event the ends of the ring-shaped conductor 15 do not join,but (as shown in inset A) are separated by a space filled with substancesuch as barium titanate 16. The barium titanate filled space 16 ispreferably of appreciable area and thin. As is well known, theelectrical polarization of the barium titanate 16 is a nonlinearfunction of the electric field acting on it. Accordingly therefore, theassembly shown in FIG. 2 radiates energy at frequencies other than thoseimposed on it when it is energized by a magnetic vector 17 correspondingwith a cyclical variation of magnetic intensity in the directionindicated. If a single frequency F is imposed as a result of inductionfrom the electromagnetic induction in the ring-shaped electricalconductor 15, the frequencies which are generated as a result, and whichmay therefore be radiated, are 2F, 3F, 4F, and so on. If the magneticvector 17 comprises electromagnetic energy at two frequencies F and Fand, particularly, if these are of approximately equal intensity, thenonlinear behavior of the barium titanate layer 16 results in thegeneration of frequencies such as F F F F 2F F F, 2B, and various othercombinations of sums and differences of multiples of the frequencies Fand F The contraband marker element 2 (FIG. 1) may be composed of thestructure such as I have described in my FIG. 2. In the previousdescription of FIG. 2, I have recited only the essential components,those which pertain to its electrical and signal inducing behavior, bywhich it serves to identify merchandise 1 (FIG. 1) that is stolen. Acheckout stand deactivating arrangement can be employed in the generalmanner shown at reference numeral 4 in FIG. 1. In the use of thecontraband marker element 2 of the type set out in FIG. 2, thedeactiviating device 4 (FIG. 1) comprises an electromagnetic energysource which radiates, at least sometimes, electromagnetic energycorresponding with the frequency of mechanical resonance of the bariumtitanate mass 16 and the nearby portions of the attached metalring-shaped conductor 15. In this form of dactivation, the energy ofmechanical vibration induced by the deactivating device 3 (FIG. I)fractures the barium titanate mass M, thus destroying or noticablychanging the behavior of this type of electromagnetic marker. By thischange I can recognize that the contraband marker element 2 (FIG. I) wasdeactivated, and therefore determine that the attached merchandise 1(FIG. 1) was sold.

Another technique of deactivation which may be employed in connectionwith the FIG. 2 device makes use of the constriction of the ring-shapedconductor 15 at the point If as shown in the inset B. In deactivating, Imay, if I choose, make use of this constriction 18 by inducing on thering-shaped conductor 15 enough current to melt or destroy theelectrically conducting material present at the constriction 18. If itis desired to make the constriction 18 sensitive and easily destroyed,the material present in the constriction 18 may, in fact, be composed ofa substance or substances less well adapted to conduct electricity thanis the main portion of the ring-shaped conductor 15. By such a choice,heat or other alteration will occur readily at the constriction 18causing the circuit involving the ring-shaped conductor l5 and thebarium titanate mass 16 to open up with the result that the contrabandmarker element 2 (FIG. 1) will no longer function to produce summationand difference frequencies, and therefore is not detected by theenergizing and detecting system 5 (FIG. 1) of the outgoing doorway 6 ofFIG. I.

I turn now to FIG. 3, in which I illustrate a further variation ofcontraband marker element 2 (FIG. 1). In inset A of FIG. 3 I show aconductor 19 and a mass of barium titanate at a separation between theends of the conductor 19, as shown in inset B. In addition, a quantityof ferromagnetic material 21 (shown in an inset B) is disposed in such amanner that it closes a magnetic circuit surrounding the current flowingin the conductor 19 in a manner to substantially increase the inductanceexhibited by the one turn loop of the conductor 19. As a result of theuse of ferromagnetic material 21 and because of the relatively largeelectrical capacity of the gap containing the barium titanate 20 (ascompared with a gap containing ordinary dielectric) the systemillustrated in this figure is, in fact, and inductance and capacitanceloop which, because of the ordinary considerations of communicationsengineering has a resonance frequency of:

where L is inductance in microhenries C is capacitance in microfarads Inview of the presence of the ferromagnetic material, the above describedresonance is not as sharp as resonances of air core coils containinglarge amounts of electrically conductive material but containing noferromagnetic material. For the reason that the resonance of systemdescribed in FIG. 3 has an appreciable width, I can, if I choose,energize it with more than one frequency, the said frequencies differingappreciably, and yet expect that both frequencies will lie substantiallywithin the resonance. The operation of my system as set out in FIG. 1,employing contraband marker elements 2 (FIG. I) but of the special typeprovided in FIG. 3 works in a manner generally similar to thedescription I have given in my discussion of the operation of my systemwith the contraband marker element of FIG. 2, but will require that theenergy sources at the energizing and detecting systems 5 (FIG. 1) of theoutgoing doorway 6 (FIG. 1) supply frequencies falling within thecapacity and inductance resonance of the system for the greatestefficiency of energy delivery to the marker. The vector arrow 17 throughthe center of the loop formed by the conductor 19 has exactly the samesignificance as the vector arrow 17 in FIG. 2.

In FIG. 4 I illustrate a contraband marker comprising a flat coil of oneor more turns short circuited on itself. An equivalent to theillustrated flat coil 22 would be, for instance, a copper washeroccupying the same region of space, and having in it an amount of copperequal to the total amount contained in the wire of the illustrated coil22. Ferromagnetic elements 23 and 24 (shown in the inset) are disposedto link the magnetic flux developed by the coil 22, and are chosen ofmaterial of extremely low magnetic coercive force. Additionally theferromagnetic elements 23 and 24 are deliberately taken in a form havingan insufficient amount of ferromagnetic material, creating a stronglikelihood that magnetic saturation will occur. By the occurrence ofmagnetic saturation, which is a nonlinear process, the flux changesassociated with the nonlinearity cause radiation from the electricallyconducting loop 25. The frequencies so radiated correspond withmodulation products, serving the same purposes as the modulationproducts developed in connection with the uses of my other contrabandmarker elements 2 (FIG. 1).

The ferromagnetic elements 23 and 24 are shown in a form adapted toserve the purpose of linking the magnetic flux induced in the presenceof my short circuited coil 22, but do not have to be bent sharply to goaround the flat coil 22. Instead I lay a piece 24 flatwise immediatelybelow the flat coil 22 and another piece 23 similarly above it. The twopieces 23 and 24 approach each other very closely at their extremities,permitting the easy transfer of magnetic flux from one piece into theother, thus allowing the circulation of magnetic flux around theconductor. The marker element illustrated in FIG. 4 is not provided withany deactivation capabilities. Instead the user has to remove this typeof marker labe from the merchandise at the time of sale. This markerelement of FIG. 4 is described for the purpose of illustrating how asystem not involving deactivation can be combined in my invention. Inuse, the energizing and detecting system 5 (FIG. II) in the vicinity ofthe outgoing doorway 6 (FIG. 1) produces and detects from thiscontraband element (FIG. 4) a signal showing that the merchandise 1(FIG. 1) being taken out still has the contraband marker element 2(FIG. 1) on it. Merchandise from which the clerk has removed thecontraband marker element of FIG. 4, of course, does not give thiseffect at the doorway.

In FIG. 5A, I show another type of contraband marker elementcorresponding with a longitudinally extending strip or rod offerromagnetic material 26 capable of responding at the frequencies F 1and F 2 delivered at my energizing and detecting system 5 (FIG. 1) inthe vicinity of the outgoing doorway 6 (FIG. 1). Linking the equatorialregion of the longitudinally extending ferromagnetic material 26 Iprovide an electrical conductor 27 which makes one or more turns aroundthe equatorial region of the said ferromagnetic material 26. Theterminations of the electrical conductor 27 are connected togetherthrough a nonlinear electric element 28 comprising a germanium rectifierjunction, a copper oxide rectifier junction, a silicon rectifierjunction, or suitable other more or less unilaterally electricallyconducting arrangement. In use, the knee of the voltage versus currentcharacteristic for the nonlinear elements represents a nonlinearitywhich imposes its effect on any current induced in or flowing throughthe electrical conductor 27. The nonlinear effect thus imposed reacts onthe magnetic field in the ferromagnetic element 26 causing summation anddifference frequencies to be magnetically radiated, as is the case withthe contraband marker elements 2 (FIG. I) previously described. Theutilization of summation and difi'erence frequencies is, in fact, thesame. Deactivation is produced at the deactivating device 4 (FIG. 1) byinducing through the diode element a large enough electrical current todestroy it. In the destroyed form, the nonlinear element either losesits directional characteristic, which removes the nonlinear behavior, oron the other hand, it may break up and become an open circuit, resultingin the passage of no current at all in the electrical conductor 27thereby removing the nonlinear effect originally present due to thenonlinear electrical element 2%.

Referring further to FIG. 5A, I may, if I choose, employ a nonlinearelement sufficiently durable that it can resist the work of mydeactivating device 4 (FIG. 1)

which I provide in the checkout stand area 3 (FIG. 1). In this case, theutilization of my system proceeds through the removal of the contrabandmarker element of FIG. A by the clerk at the checkout stand area 3 (FIG.I) at the time merchandise is purchased. Otherwise the system functionsgenerally in the same manner as it does in connection with my othercontraband marker devices.

In a further modification of my FIG. 5A marker element which I haveillustrated as 58, I provide the same longitudinally extendingferromagnetic element 26, the same electrical conductor 27, extendingone or more turns around the girth of the longitudinally extendingferromagnetic element 26 in the vicinity of its equator, but the diodeor nonlinear electrical conductor 28 which I afforded in my FIG. 5A ismodified (in FIG. 5B) to comprise, instead, two diodes 29 and 30connected in parallel, and aiding. The two diodes 29 and 30 are disposeddifferently, the diode 29 having a much larger electric current carryingcapacity than the other diode 30. The much larger current capacity ofthe diode 29 is so chosen that the deactivating device 4 (FIG. 1)operating in the checkout stand area 3 (FIG. 1) cannot cause enoughelectric current to flow in the conductor 27 to damage the diode 29. Onthe other hand, the diode 30 which has less current carrying capacity isdestroyed. In addition, to cause the electric current delivered by theconductor 27 to be shared in a predetermined manner between diodes 29and 30, I also provide resistors R and R each in series with thecorresponding diodes 29 and 30.

In use, the effect of the modified form 5B marker device is thatdeactivation between deactivating device 4 (FIG. I) employed in thecheckout stand area 3 (FIG. 1) results in a predetermined andpredictable change in the properties of the contraband marker elementcorresponding with this figure, but leaves it still able to deliver aradiation effect corresponding with modulation products, at the doorwayarea. Like my other contraband marker corresponding with FIG. 2 thiscontraband marker element as shown in FIG. 5B affords recognition ofstolen merchandise and at the same time affords, at the outgoingdoorway, recognition of the fact that contraband marking, in deactivatedform, is present on the merchandise being carried by the customerthrough the outgoing doorway 6 (FIG. 1).

Attention is now directed to the energizing and detecting system 5(FIG. 1) situated in the outgoing doorway 6 (FIG. 1). Because there arethree perpendicular coordinates available in space of three dimensions,two energizing systems and detecting devices can be arranged to work ina non-interacting manner. In fact, it is a characteristic of oneembodiment of the invention that within the limits of accuracy ofadjustment of the position and orientation of the electromagneticradiating and receiving components, the two radiating components radiateindependently, neither one being capable of transmitting energy into theother one, and further, the detecting or receiving pickup does notreceive energy directly from either of the radiating devices. Thesearrangements of course are valid only when the space in the doorway isempty, there being no contraband marker elements 2 (FIG. l) in it. Thistype of arrangement which has been generally recited above is depictedin more detail in FIG. 6.

In FIG. 6 I have pictured two pedestals 31, each containing near itscenter a pair of sending coils 32. All the sending coils 32 areconnected in parallel (or they could have been connected in series). Forillustration only, I will suppose that the frequency by which thesesending coils 32 are energized is 21 kilohertz. Each such sending coil32 is separately tuned to exhibit the highest possible impedence at 21kilohertz. For illustration only, the coils may be composed of 99 turnsof No. 20 copper wire wound on a one inch diameter coil form in a singlelayer to produce 99 turns in a total length of 3% inches. Such a coilmay be resonated to 21 kilohertz by the use of an electrical capacity ofnot less than one microfarad and not more than 1.1 microfarad. Thecombination of one of these coils 33 with its resonating capacitor 34(as shown in the inset), when energized at the resonant frequency,represents an entirely resistive impedence and in the illustrative caseexhibits a resistance between and ohms. A parallel combination of foursuch resistive loads has a combined effect adapted to efficiently loadthe voice coil outputs of some available audio amplifiers.

Similarly, there are situated at the bottom and at the top of each ofthe pedestals 31, coils 35 intended for transmitting another chosenfrequency such as (for illustration only) 24.5 kilohertz. The four coils35 which are intended for 24.5 kilohertz radiation may be constructedsimilarly and resonated similarly, but, of course, resonate with acorrespondingly smaller electrical capacity for each coil. Thecombination of the first group of four coils 32 is connected to a sourceof electrical energy 36 at 2] kilohertz. The combination of the secondgroup of four coils 35 is connected to a separate, entirely independent,source of electrical energy 37 at 24.5 kilohertz. Because of thearrangement which I have chosen for the first group of coils 32 and forthe second group of coils 35, there is no appreciable mutual inductanceacting to deliver 21 kilohertz energy into the 24.5 kilohertz, or viceversa.

At four other locations are presented four more coils 38 with their axesperpendicular to the plane of the paper. Because all the contributionsof the first group of four coils 32 and the second group of four coils35 lie in the plane of the paper, the four coils 38 with their axesperpendicular to the plane of the paper do not receive energy neither at24.5 kilohertz, nor at 21 kilohertz. The four coils 38 with their axesperpendicular to the paper are resonated at 3.5 kilohertz by choosing anappropriate electrical capacitance. In order to achieve good sensitivityin these coils, and in order that they may be resonated efficiently atthe frequency of 3.5 kilohertz, more copper is required in the winding,preferably four layers of No. 20 wire, each layer containing 99 turnsmore or less. The capacity required to resonate such a coil is in thegeneral vicinity of two microfarads for 3.5 kilohertz.

I call attention to the fact that the cores of these windings have notbeen specified thus far. It is a preferred choice to wind them onnon-magnetic, electrically non-conducting material, for the reason thatferromagnetic material (because of its nonlinear properties) imparts tomy system undesirable interactions between the energy sources.Electrically conducting material, on the other hand, destroys thequality of the inductive performance of all the coils. As a matter offact an air core coil of 99 turns, made in the manner that I havedescribed, has a Q in the vicinity of 500 at 21 kilohertz when wound ona wooden core. The resonance cannot be found, nor the inductancemeasured well enough to determine the Q if it is wound on an electricalconductor as a core.

The combination of the four coils, as described, with their axesperpendicular to the paper (each coil resonated at 3.5 kilohertz byappropriate electrical capacitance) delivers its output to the ingoingend of a high gain tuned amplifier 39 adapted to selectively receive andamplify electrical signals at 3.5 kilohertz. The amplifier 39 deliversits output to an alarm mechanism 40, or to a carrier frequency module,which is discussed further on. To achieve a closer impedence match withrespect to the commonly prevailing input resistance of the amplifiersthat are the most convenient, I may choose to vary from the connectionsshown in FIG. 8, and connect the four receiving coils 38 (the ones withtheir axes perpendicular to the paper) in series. The resistivecomponent of these coils (with their resonators connected) comes out foreach such resonated system in the vicinity of 100 ohms, with the resultthat the series of four of them are a close match to the communicationsimpedance figure of 500 ohms, a common choice for amplifiers, filters,etc.

I turn now to FIG. 7 presented for the purpose of diagrammaticallyassisting in the explanation of the manner of functioning of theenergizing and detecting system (FIG. 1) which I have particularlydetailed and described in connection with FIG. 6. In FIG. 7 the axis Xmay be taken to represent the action of the 21 kilohertz radiator, theperpendicular axis Y illustrates the action of the 24.5 kilohertz.radiator, and the axis represents the receiving sensitivity or directionof the 3.5 kilohertz receiving coils 38 of FIG. 6. The vector 6 isillustrated in a direction not parallel to nor perpendicular to any ofthe three axes. The vector 6 represents the direction in which acontraband marker element 2 (FIG. 1) is capable of receiving andre-radiating energy. Because the vector 0 has an appreciable componentin all three axes, the contraband marker element 2 (FIG. 1) oriented inaccord with this vector is able to receive energy concurrently at 21kilohertz, and likewise at 24.5 kilohertz. For similar reasons, if thecountraband marker element 2 (FIG. 1) re-radiates at 3.5 kilohertz (notbeing deactivated) then detection axis Z is so directed with respect tothe vector 0 that the said detection system is not insensitive toradiation emitted by the contraband marker element 2 (FIG. I).

The user, considering the information presented in connection with FIG.6, and the information just pres ented in connection with FIG. 7, willrealize that the reception of a 3.5 kilohertz in my system is adistinctive and an exclusive evidence of the presence of contrabandmarker elements 2. (FIG. 1). One or more such elements must be in thedomain of energy radiation and sensitivity provided by the arrangementsshown in FIG. 6 to deliver a 3.5 kilohertz signal. Other entities thancontraband marker elements are not entirely without effect, but they donot present the same effects.

To aid the understanding of another modification of my system which Ihave described, I turn again to FIG. 7. In FIG. 7 I have represented thedirections of action of the energy source frequencies X and Y (21 and24.5 kilohertz sources) and the direction of sensitivity of the systemthat detects the difference tone Z in the form of three perpendicularaxes. To the worker skilled in the art, it is evident that if contrabandvector 0 is exactly perpendicular to either of the signal source axes Xor Y, energy is eliminated which corresponds with the vector to whichthe vector 0 is perpendicular. Furthermore, if the vector 6 lies in theX Y plane, it is perpendicular at all times to the axes 2 whichtherefore prohibits the reception of any energy in the signal receivingsystem 38, (FIG. 6). It is, in fact, true that the vector 9 must haveappreciable and comparable components or direction cosines aligned withall three of the vectors X, Y, and Z. For those directions 0 which donot fulfill these conditions, either the difference tone signals are notproduced or they are not observed (if produced) by the contraband markerelement 2 (FIG. 1). The fact that there are so many blind spots and somany requirements on the direction of contraband, causes the system,conceived as in the foregoing, to sometimes fail to recognize contrabandmarkers passing through the outgoing doorway 6 (FIG. 1). It stillremains a fact that nothing other than a contraband marker will ring thealarm. However, a way has been discovered to reduce the inconvenienceresulting from the above noted limitations (which now and then permit acontraband marked piece of stolen merchandise to get through).

The user will note in FIG. 6 that in the foregoing the energy from the21 kilohertz source has been excluded from the 24.5 kilohertz source byarranging for separate radiators, and arranging that these benoninteracting because of their perpendicularity arrangement. Anotherapproach to excluding wrong pathways of signal energy is quiteapplicable in the frequency range which I have chosen, an approach notdependent on geometry. My modification permits advantages in thesimplification of the doorway structure.

The system which is contemplated for the reduction of the number ofblind spots in respect to the direction of the vector 6 (FIG. 7)substitutes rigorously designed wave filters, containing passiveelements only. These perform the function performed by the geometricisolation in the system of FIG. 6. Such wave filters can be designed forthe range of frequency in the vicinity of 20 to 50 kilohertz without theuse of ferromagnetic material or anything elese which would impose anonlinearity. The wave filters thus used, if provided in a sufficientnumber of sections, propagate the desired energy substantially withoutloss and are able to reject the unwanted sigial frequencies to whateverextent is desired, through the use of a sufficient number of networks. Aproperly designed M or 1r derived filter network will exclude unwantedfrequencies by over one hundred decibels in just a few networks.

Lattice type filters may be employed for single frequency rejection andare extremely effective. In fact, the only serious limitation on therejection brought about by a lattice type filter is imposed by variationin frequency of the signal which it is desired to reject. A lattice typefilter, for example, may comprise two electrical capacitances and twoinductive elements as the four components of a bridge. The input to thebridge and the output to the bridge have a ratio which theoretically isinfinite at the frequency at which it balances. Thus it is theoreticallypossible to exclude a single frequency to any extent, by a singlenetwork of such a filter. At the same time a single network latticefilter can transmit very efficiently energy corresponding with signalfrequencies that are substantially different from the signal frequencyat which the bridge balances.

For 20 kilohertz or more, substantially perfect inductances (inductanceswith a Q in the realm of thoullll sands) can be delivered in the spaceof a few cubic inches, and need not contain more than an ounce or two ofcopper wire. Again, in the frequency spectrum involving a metal boxcomprised of iron or copper, and with a coil spaced from the walls,inside the box, the coil neither radiates nor absorbs electromagneticenergy appreciably in this kilohertz range. Capacitances constructed ofaluminum foil and wound with such a dielectric as wax paper (or mylar orpolystyrene) gives a substantially perfect electrical performance in mypreferred frequency range. It is, accordingly, entirely feasible tocontemplate the substitution of rigorous filtering in place of thepreviously described geometric means of arranging radiator coils so thatenergy is not transferred from one system to another. Moreover, the useof well designed filters has a further advantage, that the presence ofconducting bodies of any description in the doorway 6 (FIG. 1) does notcause energy to flow from one system to the other, since the wavefilters function independently of whatever bodies are situated in thedoorway 6 (FIG. 1). On the contrary, the geometric arrangement of coilsis sensitive to the presence of electrically conducting bodies in thedoorway 6 (FIG. 1) and the favorable results which is achieved by makingthese coils 32, 35, and 38 (FIG. 6) perpendicular are partly destroyedwhenever a large electrically conducting body passes through theoutgoing doorway 6 (FIG. 1).

I turn now to FIG. 8 which illustrates the plan comprised in a generalway in the foregoing discussion. In FIG. 8, for simplicity I illustrateone common radiating and receiving means 41, and one only, since thisshows the flexibility of my modified plan most clearly. In the blockdiagram, the user will note that there are provided three distinct wavefilters, each connected at its input to a separate electrical entity.The electrical entity to which the first two wave filters are connectedis in each instance an oscillator. For convenience, the filters 42 and43 are also designed by the symbol F, and F to indicate the center of apass band which each of the said filters 42 and 43 selectivelytransmits. The third filter 44 is designated by the symbol F, F toindicate the fact that the center of its pass band is chosen at thedifference frequencies corresponding with the difference between the twofrequencies F, and F The filters in question are deliberately taken fromdesigns which permit extremely strong selectivity and extremely highexclusion of the unwanted frequencies.

As an example of a frequency corresponding with a capability ofextremely strong filtering, F, may be 31 kilohertz, F, may be 21kilohertz, and F, F in fact, 10 kilohertz. These frequencies can be verystringently filtered against one another and, in fact, exclusivity canbe achieved to whatever extent is required. I therefore indicate theseentities as being each connected to a single electronic device in thedoorway detecting and energizing system 41. A suitable doorway sensingand detecting device 41 adapted for the purpose is a flat wound coil 41diagrammatically shown in FIG. 10. Such a flat wound coil serveseffectively because the two input energy sources 46 and 47 cause aconcurrent influence on the contraband at the frequencies F, and Fwhenever a contraband element has a significant component of its vectorin a direction not in the plane of the coil. In a completely reciprocalmanner, the illustrated doorway coil 42 is able to receive energy at thedifference tone F, F, with good efficiency, and can do so whenever thecontraband marker element 2 (FIG. 1) exhibits an appreciable componentperpendicular to the plane of the doorway (shown in FIG. 8) (at the timethe contraband element 2 [FIG. 1] is passing through the plane of thesaid doorway).

I refer again to FIG. 8. In this figure it will be noted that there isprovided two frequency sources F, and F,, and two filter systems. It isobvious that if the frequency sources which deliver energy at F, and Fare adjusted so that the frequency F, F,, and furthermore, ifl imposethe requirement that these two alternating current energy sources be inphase, then, in this degenerate case, the entire system comprising thefrequency sources delivering energy at the two frequencies F, and F, hasthe same effect as one oscillator and one filter. Accordingly thereforeI achieve the same result if I simply omit the filter F, and theoscillator 46. In a system comprised by such an omission, since F, F,,the quantity F, F, has no significance as alternating current for thereason that F, F equals zero. However, in modulation products, as hasbeen stated, earlier, one of the functions that is generated is F, F Forthe case in which F, F F, F is of course 2F.

In the modification of the system which I am now describing with thehelp of FIG. 8, the oscillator 46 and the filter 42 are omitted. Iprovide the substitution of a filter adapted to pass the frequency 2F,instead of a filter 44 (as illustrated) to pass the frequency F, F Therecognition of contraband marked merchandise by this modified system isidentically the same as has been described in the other embodiments ofmy invention. From an engineering standpoint it is required that thefilter 43 of FIG. 8, be adapted to particularly stringent rejection ofthe frequency 2F. In a lattice filter designed for single frequencyrejection elimination of the unwanted frequency 2F, from the output ofthis filter can be accomplished to more than decibels in two meshes,providing the stability of the frequency of the oscillator 47 issufficiently good. This is easily arranged by employing crystal controlto stabilize the oscillator 47. I envision the use of a temperatureinsensitive cut of the quartz crystal and, if necessary, I employ atemperature controlled environment to further improve the frequencystability of the oscillator 47. The stability of oscillators has beencontrolled within one part per bilion over long periods by the carefuluse of these techniques. Since I do not need such extreme frequencycontrol, the adequacy of the methods which I propose is quite obvious.

In the use of my anti-shoplifting systems there is a problem ofcommunicating the warning signal indicating that merchandise is beingstolen, and bringing the indication to the attention of security guardswho are not, necessarily, at the same place. To make this procedureconvenient in finished buildings where the wiring is already in place, Ipropose the use of ordinary carrier frequency signaling techniques thatare well known in the art, and proposed that the carrier frequencysignals be inserted on the electric power system.

Since my warning devices are electrically powered, it is convenient toinsert the carrier warning signal on the cord through which the powerrequirements of the system are served, making communications connectionsof a separate nature unnecessary. The electronic equipment necessary toput the carrier frequency warning message into the power cord willgenerally be a part of, or will be situated close to the other parts ofthe antishoplifting system. In fact all these things may be on the samepanel rack or may be built up in the same stack of shielded boxes, asproves convenient. I visualize such carrier frequency systems as avaluable and useful feature in combination with the other elements of myinvention. In FIG. 8, the carrier frequency module, is as desired, theelement 48.

In FIG. 8 the operator will note that there are six electricalconnections, comprising three pairs, going from the systems: (a) 46 and42, (b) 47 and 43, and (c) 48 and 45. U.S. Pat. No. 2,520,677 (Aug. 29,1950) makes a similar use of six wires in the form of three pairs, andprovides an especially effective means for filtering out the noise fromthe signal frequency F, :t F (F 1 F is used in the discussion in thispatent application). I contemplate the use of all the same means andmethods for improving the signal to noise ratio in this antishopliftingsystem, and employ the same in combination with the other features of myanti-shoplifting system to better reject unwanted noise and electricaldisturbances of all kinds.

I refer one more to FIG. 8, and particularly I employ the device of FIG.8 with the omission of elements 43, 44, 45, 47 and 48. I furtherdescribe the filter F, (element 42) as a non-significant componentcomprised in this use of FIG. 8 device as simply a pair of wires goingstraight through from left to right. In effect I omit the function ofthis filter. In this use of the FIG. 8 device I also construe theoscillator 46 as one emitting relatively very strong electricaloscillations, and one which may at times be adjusted or at least haveits frequency reset to another value as required. Further the oscillator46 may be a warble" oscillator adapted to cyclically retraverse a smallrange of frequency.

In the use which I am now describing for the FIG. 8 device, I insert thecoil identified in FIG. 8 as doorway at the point shown for the device 4in FIG. 1. The coil 41 is assumed to be taken to a proper scale so thatit will fit in the space provided at location 4 in FIG. 1. My FIG. 8device so arranged is, in fact, suitable to perform the deactivatingfunction. To assure the upward radiation of a strong electromagneticeffect through the belt 2A of the checkout stand 3 shown in FIG. 1, Iarrange the design of the checkout stand so that there are no closedmetallic loops between the device 4 and the merchandise 1 withcontraband marker 2. I further designate that the plane of my FIG. 8coil 41 will be the same as the plane of the largest side of the boxshaped space designated at numeral 4 in FIG. 1. For this use, and forall the other uses of the FIG. 8 device, it is understood that themechanical coil support which is illustrated in FIG. 8 is anelectrically non-conducting material, and a non-ferromagnetic material.

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art, and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

What is claimed is:

l. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising:

a generally ring-shaped electrical conductor including a loop having apair of ends which do not close for carrying a current induced by saidelectromagnetic field, and

a ferromagnetic substance which is nonlinearly polarized in response tosaid electromagnetic field and is electrically connected to the ends ofsaid conductor, wherein said marker radiates detectable electromagneticradiation at a predetermined frequency when disposed within saidinterrogation zone.

2. A marker according to claim 1 wherein said substance comprises bariumtitanate.

3. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising:

a generally ring-shaped electrical conductor for carrying a currentinduced by said electromagnetic field,

a substance which is nonlinearly polarized in response to saidelectromagnetic field and is electrically connected to said conductor,wherein said marker radiates detectable electromagnetic radiation at apredetermined frequency when disposed within said interrogation zone,and

the conductivity of a segment of said ring-shaped conductor beingdegradable by a predetermined current flow therethrough to permitdeactivation of said marker by inducing said predetermined current insaid conductor.

4. A marker according to claim 3 wherein said segment comprises asubstance of lower conductivity than the remainder of said conductor.

5. A marker according to claim 3 wherein said segment comprises aconstriction in said conductor.

6. A system for detecting an object in an interrogation zone comprising:

means proximate the area for producing at least one oscillatingelectromagnetic field in the zone,

a marker associated with each object to be detected for reflectingdetectable electromagnetic radiation in response to energy received fromsaid oscillating electromagneticfield, said marker including aringshaped electrical conductor for carrying a current induced by saidelectromagnetic field, and further including a substance which isnonlinearly polarized in response to an electromagnetic field and whichis connected to said conductor,

means for sensing reflected electromagnetic radiation from said markerinsaid interrogation zone, and v means for deactivating said marker topermit an authorized passage of an object through the zone withoutdetection.

7. A system according to claim 6 wherein said deactivating meanscomprises:

an electromagnetic energy source for producing electromagnetic energycorresponding with the frequency of mechanical resonance of said markerfor fracturing said marker.

8. A system according to claim 6 wherein said marker includes a segmenthaving a conductivity lower than the conductivity of the remainder ofthe conductor and wherein said deactivating means comprises means forinducing in said conductor enough current to destroy the electricalconductivity of said segment.

9. A method of detecting an object in an interrogation zone comprisingthe steps of:

providing each object to be detected with a marker comprising aring-shaped electrical conductor for carrying a current induced by anelectromagnetic field the lines of which link said conductor, and asubstance connected to the conductor and which is nonlinearly polarizedin response to an electromagnetic field,

producing an oscillating electromagnetic field in the interrogation zoneto induce a current in the conductor of a marker present in the zone toradiate detectable electromagnetic radiation,

detecting in the interrogation zone said radiation,

and

selectively deactivating said marker when it is not desired to indicatethe presence of an object in the interrogation zone.

10. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising:

a generally ring-shaped electrical conductor comprising ashort-circuited loop for carrying a current induced by saidelectromagnetic field, and

a ferromagnetic material of low coercive force formed about saidconductor, said ferromagnetic material being nonlinearly polarized inresponse to said electromagnetic field wherein said marker radiatesdetectable electromagnetic radiation at a predetermined frequency whendisposed within said interrogation zone.

1 l. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising:

an electrical conductor comprising a loop having a pair of spaced apartends for carrying a current induced by said electromagnetic field,

a nonlinearly polarized material electrically connecting the ends ofsaid loop such that said marker radiates electromagnetic radiation whendisposed within said interrogation zone, and

ferromagnetic material disposed about the outer periphery of said loopto increase the inductance of said loop, wherein said marker radiates aresonance frequency when placed within said interrogation zone.

12. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an electromagnetic field, themarker comprising:

an electrical conductor for carrying a current induced by saidelectromagnetic field,

a wire disposed around said conductor, and

a nonlinearly polarized rectifier junction connected at each end to saidwire wherein said marker radiates electromagnetic radiation whendisposed within said interrogation zone.

13. A system for detecting an object in an interrogation zonecomprising:

means proximate the area for producing at least one oscillatingelectromagnetic field in the zone,

a marker associated with each object to be detected for reflectingdetectable electromagnetic radiation in response to energy received fromsaid oscillating electromagnetic field, said marker including aringshaped electrical conductor for carrying a current induced by saidelectromagnetic field, and further including barium titanate which isnonlinearly polarized in response to an electromagnetic field and whichis connected to said conductor, and

means for sensing reflected electromagnetic radiation from said markerin said interrogation zone.

Notice of Adverse Decision in Interference In Interference No. 98,962involving Patent No. 3,754,226, E. R. Fearon, CONDUCTIVE-RINGFERROMAGNETIC MARKER AN D METHOD AND SYSTEM FOR USING SAME, finaljudgment adverse to the patentee was rendered. Dec. 23, 1975, as toclaims 3, 4, 6, 8 and 9.

[Ofiioz'al Gazette March 23, 1976.]

Notice of Adverse Decision in Interference In Interference No. 98,962involving Patent No. 3,754,226, E. R. Fearon, CONDUCTIVE-RINGFERROMAGNETIC MARKER AN D METHOD AND SYSTEM FOR USING SAME, finaljudgment adverse to the patentee was rendered. Dec. 23, 1975, as toclaims 3, 4, 6, 8 and 9.

[Ofiioz'al Gazette March 23, 1976.]

Notice of Adverse Decision in Interference In Interference No. 98,962involving Patent No. 3,754,226, E. R. Fearon, CONDUOTIVE-RINGFERROMAGNETIC MARKER AND METHOD AND SYSTEM FOR USING SAME, finaljudgment adverse to the patentee was rendered Dec. 23, 1975, as toclaims 3, 4L, 6, 8 and 9.

[Oyfioz'al Gazette March 25, 1.976.]

1. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising: a generally ring-Shapedelectrical conductor including a loop having a pair of ends which do notclose for carrying a current induced by said electromagnetic field, anda ferromagnetic substance which is nonlinearly polarized in response tosaid electromagnetic field and is electrically connected to the ends ofsaid conductor, wherein said marker radiates detectable electromagneticradiation at a predetermined frequency when disposed within saidinterrogation zone.
 2. A marker according to claim 1 wherein saidsubstance comprises barium titanate.
 3. A marker for being secured to anobject to enable detection of the object within an interrogation zonehaving an oscillating electromagnetic field, the marker comprising: agenerally ring-shaped electrical conductor for carrying a currentinduced by said electromagnetic field, a substance which is nonlinearlypolarized in response to said electromagnetic field and is electricallyconnected to said conductor, wherein said marker radiates detectableelectromagnetic radiation at a predetermined frequency when disposedwithin said interrogation zone, and the conductivity of a segment ofsaid ring-shaped conductor being degradable by a predetermined currentflow therethrough to permit deactivation of said marker by inducing saidpredetermined current in said conductor.
 4. A marker according to claim3 wherein said segment comprises a substance of lower conductivity thanthe remainder of said conductor.
 5. A marker according to claim 3wherein said segment comprises a constriction in said conductor.
 6. Asystem for detecting an object in an interrogation zone comprising:means proximate the area for producing at least one oscillatingelectromagnetic field in the zone, a marker associated with each objectto be detected for reflecting detectable electromagnetic radiation inresponse to energy received from said oscillating electromagnetic field,said marker including a ring-shaped electrical conductor for carrying acurrent induced by said electromagnetic field, and further including asubstance which is nonlinearly polarized in response to anelectromagnetic field and which is connected to said conductor, meansfor sensing reflected electromagnetic radiation from said marker in saidinterrogation zone, and means for deactivating said marker to permit anauthorized passage of an object through the zone without detection.
 7. Asystem according to claim 6 wherein said deactivating means comprises:an electromagnetic energy source for producing electromagnetic energycorresponding with the frequency of mechanical resonance of said markerfor fracturing said marker.
 8. A system according to claim 6 whereinsaid marker includes a segment having a conductivity lower than theconductivity of the remainder of the conductor and wherein saiddeactivating means comprises means for inducing in said conductor enoughcurrent to destroy the electrical conductivity of said segment.
 9. Amethod of detecting an object in an interrogation zone comprising thesteps of: providing each object to be detected with a marker comprisinga ring-shaped electrical conductor for carrying a current induced by anelectromagnetic field the lines of which link said conductor, and asubstance connected to the conductor and which is nonlinearly polarizedin response to an electromagnetic field, producing an oscillatingelectromagnetic field in the interrogation zone to induce a current inthe conductor of a marker present in the zone to radiate detectableelectromagnetic radiation, detecting in the interrogation zone saidradiation, and selectively deactivating said marker when it is notdesired to indicate the presence of an object in the interrogation zone.10. A marker for being secured to an object to enable detection of theobject within an interrogation zone having an oscillatingelectromagnetic field, the marker comprising: a generally ring-shapedelectrical conductor comprIsing a short-circuited loop for carrying acurrent induced by said electromagnetic field, and a ferromagneticmaterial of low coercive force formed about said conductor, saidferromagnetic material being nonlinearly polarized in response to saidelectromagnetic field wherein said marker radiates detectableelectromagnetic radiation at a predetermined frequency when disposedwithin said interrogation zone.
 11. A marker for being secured to anobject to enable detection of the object within an interrogation zonehaving an oscillating electromagnetic field, the marker comprising: anelectrical conductor comprising a loop having a pair of spaced apartends for carrying a current induced by said electromagnetic field, anonlinearly polarized material electrically connecting the ends of saidloop such that said marker radiates electromagnetic radiation whendisposed within said interrogation zone, and ferromagnetic materialdisposed about the outer periphery of said loop to increase theinductance of said loop, wherein said marker radiates a resonancefrequency when placed within said interrogation zone.
 12. A marker forbeing secured to an object to enable detection of the object within aninterrogation zone having an electromagnetic field, the markercomprising: an electrical conductor for carrying a current induced bysaid electromagnetic field, a wire disposed around said conductor, and anonlinearly polarized rectifier junction connected at each end to saidwire wherein said marker radiates electromagnetic radiation whendisposed within said interrogation zone.
 13. A system for detecting anobject in an interrogation zone comprising: means proximate the area forproducing at least one oscillating electromagnetic field in the zone, amarker associated with each object to be detected for reflectingdetectable electromagnetic radiation in response to energy received fromsaid oscillating electromagnetic field, said marker including aring-shaped electrical conductor for carrying a current induced by saidelectromagnetic field, and further including barium titanate which isnonlinearly polarized in response to an electromagnetic field and whichis connected to said conductor, and means for sensing reflectedelectromagnetic radiation from said marker in said interrogation zone.